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Concerted Spin Crossover and Symmetry Breaking Yield Three Thermally and One Light-Induced Crystallographic Phases of a Molecular Material.

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DOI: 10.1002/ange.200904190
Spin Crossover
Concerted Spin Crossover and Symmetry Breaking Yield Three
Thermally and One Light-Induced Crystallographic Phases of a
Molecular Material**
Nicolas Brfuel , Hiroshi Watanabe, Loic Toupet, Jrmy Come, Naohide Matsumoto,
Eric Collet,* Koichiro Tanaka, and Jean-Pierre Tuchagues*
Among switchable molecular materials, FeII spin-crossover
(SC) complexes have been widely studied over the last
decades.[1] Their reversible low-spin (LS)Ðhigh-spin (HS)
switching triggered by a change in temperature or pressure, or
by light irradiation, has attracted much interest for both basic
scientific understanding and potential technological applications in information storage or visual displays.[2, 3] In these
materials, the coexistence of short- and long-range interactions between molecules yields cooperative effects such as
hysteresis and/or two-step transitions. Usually, the SC phenomenon is isostructural, but in a few cases symmetry
breaking occurs in the LS phase, alongside intermolecular
reorganization.[4, 5] Few examples of mononuclear molecular
materials that undergo two-step SC associated with intermolecular reorganization in the broken-symmetry phase, the so[*] Dr. L. Toupet, Prof. E. Collet
Institut de Physique de Rennes, UMR CNRS 6251
Universit Rennes 1, 263 av. Gnral Leclerc
35042 Rennes cedex (France)
Fax: (+ 33) 223-236717
php?who = 342&lg = EN
Dr. N. Brfuel ,[+] J. Come, Prof. J.-P. Tuchagues
CNRS; LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse (France)
Universit de Toulouse; UPS, INPT; LCC
F-31077 Toulouse (France)
Fax: (+ 33) 561-553003
called intermediate (INT) phase, involving fractional population of the HS state, have been reported.[6, 7] To date, the
INT phase has been fully described only in very few cases by
diffraction techniques, which evidenced HS–LS[8, 9] or LS–HS–
LS[10] long- or short-range ordering.[6] Over the last ten years,
we have investigated supramolecular FeII SC materials
including imidazolyl groups.[5, 8, 9, 11] In view of the first-order
SC evidenced in [FeIIH2L2Me](ClO4)2,[5] where H2L2Me denotes
the acyclic hexadentate N6 Schiff base bis[N-(2-methylimidazol-4-yl)methylidene-3-aminopropyl]ethylenediamine,
new SC material [FeIIH2L2Me](PF6)2, 1 has been synthesized.
Here we report on its two rare
types of behavior: long-range LS–
HS–HS–LS ordering in the INT
phase, and structural symmetry
breaking in the LS phase. In addition, another symmetry-breaking
process occurs on generating the
photoinduced HS phase (PIHS): in
the emerging field of photoinduced
phase transitions,[12] this result opens a new subject of debate,
that is, the possibility of reaching different types of false
ground states through light irradiation.
Figure 1 shows the thermal variation of the cM T product
of 1 (cM is the molar magnetic susceptibility), in the cooling
and warming modes, evidencing a two-step SC process. The
Dr. N. Brfuel ,[+] Prof. N. Matsumoto
Department of Chemistry, Kumamoto University (Japan)
H. Watanabe, Prof. K. Tanaka
Department of Physics, Kyoto University (Japan)
Institute for Integrated Cell-Material Sciences
Kyoto University (Japan)
[+] Present address: Laboratoire National des Champs Magntiques
Intenses, UPR 3228, 25 Rue des Martyrs, 38042 Grenoble cedex 9
[**] This work was supported by Grants in Aid for Scientific Research
(No. 16205010) from the Ministry of Education, Science, Sports,
and Culture (Japan), Grants in Aid for Creative Scientific Research
Program (No. 18GS0208) from the Ministry of Education, Science,
Sports, and Culture (Japan), the Institut Universitaire de France,
and a Grant “CREATE Ultimate” No. 4146 from Rgion Bretagne,
France. N.B. is grateful to the JSPS for providing a Foreign
Postdoctoral Fellowship.
Figure 1. Temperature dependence of cM T in the 10–300 K range for 1
measured first on cooling ( ! )and then on warming mode (~), at
1 K min1 sweeping rate; (~) indicates the temperature dependence of
cM T at the same sweep rate after irradiating the sample at 10 K with a
532 nm laser.
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2009, 121, 9468 –9471
cM T value is about 3.4 cm3 mol1 K at 300 K (HS phase) and
continuously decreases to reach a pseudoplateau with a width
of about 30 K centered at about 120 K, where cM T
1.5 cm3 mol1 K, the value expected for an approximately
50 % distribution of the HS and LS states (INT phase). The
INTÐLS step centered around 97 K shows a thermal
hysteresis loop of 6 K width, characteristic of a first-order
When 1 was irradiated with green continuous-wave light
(532 nm, 1 mW m2), a light-induced excited spin-state trapping effect (LIESST)[13] was clearly observed. The cM T
product increased from an initial value of 0.02 cm3 mol1 K
to a saturation regime at 3.4 cm3 mol1 K over one hour, which
indicates quantitative photoconversion from the LS to a PIHS
state. When irradiation was stopped, two relaxation steps
were observed on increasing T (ca. 1 K min1): an abrupt drop
of cM T to about 0.5 cm3 mol1 K at 50 K is followed by a
slower decrease up to 73 K, where the LS state is fully
reached. This behavior was confirmed on different samples.
The crystal structures of 1 were determined by singlecrystal X-ray diffraction[14] at 250 K (HS), 110 K [(INT
(1=2 HS + 1=2 LS)], and 80 K (LS). The nature of the photoinduced HS state at 15 K (PIHS) was also investigated by Xray diffraction after laser excitation at 532 nm. The structural
determinations showed acentric space groups with identical
[FeIIH2L2Me]2+ complex cations in which FeII assumes an
octahedral coordination environment including the six N
donors of the hexadentate ligand, two FeN(imine), two Fe
N(amine), and two FeN(imidazolyl). Figure 2 a illustrates
how H2L2Me wraps the FeII metal center to yield the
[FeIIH2L2Me]2+ cation.
The prominent features of the 250 K structure[14] (HS
state) result from location of the iron center on a twofold
symmetry axis. In the P22121 (Z = 2) space group,[15] the
asymmetric unit is made of half a [FeIIH2L2Me]2+ cation and
one PF6 anion. The average bond length (hFeNi = 2.190 )
is typical of an HS FeII site with six N donors.[1] As shown in
Figure 2 a, the crystal structure is made of cation layers and
anion layers in the ab plane alternating along the c axis,
alongside one another and connected through hydrogen
The X-ray data clearly indicate ordering of spin-states
among the FeII sites in the INT phase. The associated doubling
of the crystalline c axis is characterized by the appearance of
numerous, but weak, Bragg reflections (Figure 3) in the 97–
142 K temperature range. In addition, the space group of the
INT phase decreases to the nonisomorphic monoclinic
subgroup P21.[14] We could not detect significant deviations
of the b angles from 908, possibly because of domain
formation, which may be associated with the resulting small
decrease in crystal quality.[16]
There are two nonequivalent cation sites (four per unit
cell) in the INT phase (Figure 2 b). One is mainly HS (hFe1
Ni = 2.13(1) ), and the other one mainly LS (hFe2Ni =
2.04(1) ). Noteworthily, the hFe1Ni distance is shorter
than the hFe(HS)Ni distance at 250 K (2.190 ) while the
hFe2Ni distance is longer than hFe(LS)Ni distance at 80 K
(2.012 ). This suggests that the ordering does not correspond
to 100 % of HS (respectively 100 % of LS) and 0 % of LS
Angew. Chem. 2009, 121, 9468 –9471
Figure 2. a) Projection of the crystal packing in the HS phase of 1 at
250 K, in the ac plane and ORTEP of the cation with atom numbering
scheme. H atoms are omitted for clarity, and thermal ellipsoids are
drawn at 50 % probability. Similar projections in the ac plane are
shown for the INT phase (b) with HS (blue) and LS (red) sites, for the
LS phase (c), and for the photoinduced HS phase at 15 K (d).
Additional projections along the multiplied crystal axes on the right
show the motions of the ions.
(respectively 0 % of HS) at site 1 (respectively 2), as is often
observed in this type of symmetry-breaking phase transition
associated with ordering phenomena.[7] The bond lengths,
intermediate between those of the LS and HS states, allow a
rough estimate of the HS fraction on sites 1 (2) to be 75
(25) %. As clearly observed in Figure 2 b, this particular
packing allows structural reorganization through displacement and collective molecular rotation within each layer, and
out of phase from one layer to another. The ordering in the
INT phase results in an LS–HS–HS–LS pattern, in which two
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Reconstructed diffracted intensity in the a*c* reciprocal
plane. With respect to the HS reciprocal lattice at 250 K, new peaks
appear at l + 1/2 in the INT phase, at h + 1/4, + 1/2, + 3/4 in the LS
phase, and at l + 1/3, + 2/3 in the PIHS phase.
adjacent HS cation layers alternate along the c axis with two
adjacent LS cation layers.
Such ordering phenomena have been observed in different SC systems with different types of intermediate states
(HS–LS, HS–LS–LS),[7, 10] and result from competition
between different intrasublattice ferromagnetic-like interactions and intersublattice antiferromagnetic-like interactions.[17] These phenomena were recently discussed on the
basis of the universal Landau theory of phase transitions:[18]
the ordering is associated with the symmetry-breaking order
parameter y characterizing the difference in HS population of
the two sublattices y = (gHS1gHS2),[18] and in the present case
y 0.5.
Below 97 K, another structure appears in the P212121
space group,[14] corresponding to the LS phase, in which the
unit cell has changed to (4a,b,c) (Figure 3). The LS structure
(Figure 2 c) includes two independent LS cations (Fe1 and
Fe2 sites) per unit cell: hFe1Ni = 2.012 = hFe2Ni are
typical of LS FeII. Because of symmetry breaking, the cations
are no longer located on a twofold symmetry axis: a related
distortion accompanied with slight tilts and displacements of
the ions occurs (Figure 2 c).
The present sequence of phases P22121 (HS), P21 (1=2 HS +
=2 LS), P212121 (LS) is therefore not “re-entrant”, and differs
significantly from the already reported materials exhibiting
an intermediate ordered phase[7–10] for which the LS and HS
structures are isostructural. In addition, the different translation symmetries (a,b,2c) and (4a,b,c) forbid any group/
subgroup relation between the INT and LS phases. This
reconstructive phase transition[19] must be a first-order
process, in good agreement with the thermal hysteresis
observed around 97 K.
A novel structural reorganization occurs on generation of
the photoinduced HS state by photoexcitation at 532 nm. The
translational symmetry of this PIHS state is different from
those of the HS, INT, and LS phases with an (a,b,3c) unit cell
(Figure 3). The P22121 space group of the PIHS phase[14] is an
isomorphic subgroup of lower index of the HS phase.[16] The
PIHS structure includes two independent HS complex cations
per unit cell (Figure 2 d), one of which is located on a twofold
{[FeIIH2L2Me]2+}/2). The average FeN bond lengths of both
FeII sites (hFe1Ni = 2.17(1), hFe2Ni = 2.18(1) ) are typical of HS FeII, but the molecular distortion or rotation
concerns only the layers which are not located on the twofold
symmetry axis (Figure 2 d). The symmetry breaking occurring
in this PIHS state (triple c axis) also involves rotations and
displacements of the ions. To our knowledge this is the first
case of photoinduced spin crossover involving symmetry
The existence of different competing ground states or
false ground states is a topic of current interest,[12] especially
in molecular solids and in the emerging field of photoinduced
phase transitions. In this respect, if the HS fraction could be
stabilized between 250 and 15 K, a phase transition related to
the symmetry breaking (due to ion reorganization) would
occur, with change of lattice from (a,b,c) to (a,b,3c). However,
temperature balances the relative stability of the different
phases, and the LS phase is the true ground state. The unusual
bistability of SC compounds allows a metastable HS state to
be generated by light irradiation at low temperature. It is
demonstrated here that this false HS ground state, otherwise
inaccessible under thermal equilibrium conditions, has a
symmetry different from that of the HS phase stable at high
These results clearly indicate exceptionally strong coupling between the electronic and structural degrees of freedom of 1. The occurrence of four different phases with
different symmetries, schematically shown in the table-ofcontents figure, is unique and should be related to the strong
intermolecular interactions and to the specific packing of
anion and cation layers. The outstanding result gained from
this study is revealing that light allows another type of false
ground state to be reached through symmetry breaking,
owing to the LIESST effect.
Experimental Section
The H2L2Me ligand was prepared as previously described[5] and was
deoxygenated prior to reaction with iron(II), which was carried out in
a purified nitrogen atmosphere inside a glove box. 1 was obtained as
dark-orange crystals as follows: a solution of FeIICl2·4 H2O (0.198 g,
1 mmol) in ethanol (5 mL) was added to a solution of H2L2Me
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2009, 121, 9468 –9471
(1 mmol) in ethanol (10 mL) with stirring and heating (ca. 45–50 8C).
After 30 min, a solution of NaPF6 (0.340 g, 2 mmol) in ethanol
(10 mL) was poured drop by drop into the reaction mixture. The
mixture was stirred and heated for a further 1 h and filtered. The
filtrate was allowed to stand for several days, and the orange crystals
that formed were collected by filtration. X-ray quality dark orange
crystals were obtained in 75 % yield (0.530 g) Elemental analysis (%)
calcd for 1 (FeC18H30N8P2F12, 704.3 g mol1): C 30.70, H 4.29, N 15.91;
found: C 30.48, H 4.42, N 16.05, IR (KBr): n = 1630 (nC=N), 843 cm1
Received: July 28, 2009
Published online: November 5, 2009
Keywords: iron · magnetic properties · N ligands ·
phase transitions · spin crossover
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[14] X-ray diffraction experiments were performed on single crystals
with an Xcalibur 3 four-circle diffractometer (Oxford Diffraction). We used an Oxford Cryosystems nitrogen-flow cryostat for
measurements down to 80 K, and an Oxford Diffraction Helijet
Angew. Chem. 2009, 121, 9468 –9471
helium-flow cryostat for measurements at 15 K. The unit-cell
parameters and data reduction were obtained with the CrysAlis
software from Oxford Diffraction. The structures were solved by
direct methods (SIR-97) and refined against F2 by full-matrix
least-squares techniques (SHELXL-97) with anisotropic displacement parameters for non-hydrogen atoms (for details, see
supplementary crystallographic data). Crystal data collection
and refinement parameters: 1-250 K: MoKa radiation (l =
0.71073 ), C18H30N8P2F6Fe, Mr = 704.27, crystal dimensions
0.20 0.10 0.10 mm, orthorhombic, space group P22121, a =
8.405(1), b = 9.469(2), c = 17.399(3) , V = 1384.7(4) 3, Z (cations/unit cell) = 2, 1cald = 1.689 g cm3, m = 0.767 mm1, 21 628
reflections collected, 4094 independent reflections, wR(all
data) = 0.075, R(all data) = 0.042; 1-110 K: monoclinic, space
group P21, a = 8.185(1), b = 9.390(8), c = 35.543(2) , b =
90.011(7)8, V = 2655(2) 3, Z (cations/unit cell) = 4, 1cald =
1.762 g cm3, m = 0.800 mm1, 64 948 reflections collected,
11 446 independent reflections, wR(all data) = 0.13, R(all
data) = 0.13; 1-80 K: orthorhombic, space group P22121, a =
32.532(3), b = 9.424(1), c = 17.054(1) , V = 5228.5(8) 3, Z
(cations/unit cell) = 8, 1cald = 1.789 g cm3, m = 0.813 mm1,
123 685 reflections collected, 11 341 independent reflections,
wR(all data) = 0.051, R(all data) = 0.042; 1-15 K: orthorhombic,
space group P22121, a = 8.226(2), b = 9.330(1), c = 52.20(1) ,
V = 4006(4) 3, Z (cations/unit cell) = 8, 1cald = 1.751 g cm3, m =
0.795 mm1, 93 486 reflections collected, 8666 independent
data) = 0.174,
data) = 0.167.
CCDC 714016 (1-250 K), 714015 (1-110 K), 714014 (1-80 K)
and 736427 (1-15 K) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
Because symmetry breaking results in the monoclinic INT phase,
the nonconventional space group P22121 was chosen for the HS
phase in order to retain the monoclinic b axis.
The INT phase is associated with an order/disorder, which would
require treating the structures as the weighted contribution of
HS and LS states on each independent site. However, due to the
limited spatial resolution of the experiments, (ca. 0.8 ) this
disorder can not be solved, because the atomic displacements
between HS and LS states are small (0.2 ). The resulting
pseudosymmetry yields high correlations in the refinements
through nonpositive-definite thermal parameters in the structures with multiplied cell (INT, LS, and PIHS). The weak new
reflections as well as the decrease in crystalline quality resulting
from phase transitions may be at the origin of the R factors.
Internal R factors are 0.053 (LS), 0.037 (INT), 0.030 (HS), and
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2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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